Methanol is used as a feed stock for a variety of chemical manufacturing processes. One process that is more recently being developed is the conversion of methanol to olefin products, particularly products containing the olefins ethylene and propylene. The olefins produced from the methanol conversion process are of suitable quality to be used in polymer manufacturing processes. Of particular commercial concern in the methanol conversion process, however, is whether sufficient quantities of prime olefins (i.e., ethylene and propylene) can be produced.
U.S. Pat. No. 4,677,242 (Kaiser) describes the use of a silicoaluminophosphate (SAPO) molecular sieve catalyst for converting various oxygenates, such as methanol, to olefins. According to the patent, the SAPO catalyst is an extremely efficient catalyst for the conversion of oxygenates to prime olefin products when the feed is converted in the presence of a diluent. The diluent used has an average kinetic diameter larger than the pores of the SAPO molecular sieve. The selected SAPO molecular sieves have pores that an average kinetic diameter characterized such that the adsorption capacity (as measured by the standard McBain-Bakr gravimetric adsorption method using given adsorbate molecules) shows adsorption of oxygen (average kinetic diameter of about 3.36 angstroms) and negligible adsorption of isobutane (average kinetic diameter of about 5.0 angstroms).
U.S. Pat. No. 6,316,683 (Janssen et al.) describes a method for making an olefin product from an oxygenate feedstock while protecting the catalytic activity of a silicoaluminophosphate molecular sieve used for catalyzing the reaction. Prior to use, the molecular sieve is protected by shielding with a template molecule or by carbonaceous material on the surface of the molecular sieve material. After removing the template or carbonaceous material to activate the molecular sieve, catalytic activity is protected by maintaining the temperature of the molecular sieve above 150° C. Alternatively, the activated catalyst can be exposed to temperatures below 150° C. by preventing exposure of catalyst active sites to water.
U.S. Pat. No. 6,166,282 (Miller) describes a method for making an olefin product from an oxygenate feedstock. The oxygenate feedstock is exposed to a catalyst bed that facilitates the reaction. During the reaction, a carbonaceous product builds up on the catalyst particles. The catalyst particles are passed through a regenerator to remove the carbonaceous product.
What is desired are methods that allow for more efficient conversion of oxygenates to olefins. In particular, methods are desired that allow for protection of catalyst used in such conversions during periods of extended use.